Proppants containing dispersed piezoelectric or magnetostrictive fillers or mixtures thereof, to enable proppant tracking and monitoring in a downhole environment

ABSTRACT

In one aspect, the invention relates to a method for “tagging” proppants so that they can be tracked and monitored in a downhole environment, based on the use of composite proppant compositions containing dispersed fillers whose electromagnetic properties change at a detectable level under a mechanical stress such as the closure stress of a fracture. In another aspect, the invention relates to composite proppant compositions containing dispersed fillers whose electromagnetic properties change under a mechanical stress such as the closure stress of a fracture. The currently preferred embodiments use substantially spherical thermoset nanocomposite particles where the matrix comprises a terpolymer of styrene, ethylvinylbenzene and divinylbenzene, a PZT alloy manifesting a strong piezoelectric effect or Terfenol-D manifesting giant magnetostrictive behavior is incorporated to provide the ability to track in a downhole environment, and carbon black particles possessing a length that is less than 0.5 microns in at least one principal axis direction may optionally be incorporated as a nanofiller.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/217,735, filed Aug. 25, 2011, now allowed, which is acontinuation of U.S. patent application Ser. No. 12/206,867, filed Sep.9, 2008, now granted as U.S. Pat. No. 8,006,754, which in turn claimspriority benefit from U.S. Provisional Application Ser. No. 61/042,727,filed Apr. 5, 2008, (now expired), all of which are incorporated byreference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to a new method for “tagging” proppants sothat they can be tracked and monitored in a downhole environment. Thismethod is based on the use of new composite proppant compositionscontaining dispersed fillers whose electromagnetic properties changeunder a mechanical stress such as the closure stress of a fracture.These changes of electromagnetic properties are detected to track andmonitor the locations of the proppants.

BACKGROUND

Proppants are solids, such as sand, ceramic, polymer, or compositeparticles, that are often used during fracture stimulation to keep afracture open by resisting the closure stress applied by the geologicalformation above the fracture.

In many situations, a substantial portion of the proppant does notremain in a fracture where it has been placed but instead flows back tothe wellbore, so that it is valuable to be able to assess the extent ofany flowback. Furthermore, a knowledge of the locations of the proppantparticles can also provide valuable information about the fracturegeometry. The ability to monitor the locations of the proppant particlesover time after their placement in a downhole environment is, therefore,a highly desirable objective. Progress towards the attainment of thisobjective has hitherto been both difficult to make and limited in itsscope.

The patent to Ayoub et al. (U.S. Pat. No. 7,082,993), assigned toSchlumberger Technology Corporation, provides for a “Means and Methodfor Assessing the Geometry of a Subterranean Fracture During or After aHydraulic Fracturing Treatment”. Disclosed therein is a method fordetermining the geometry of a hydraulic fracture where said geometry canbe inferred from the use of a mixed proppant composition comprised offerrous/magnetic fibers embedded in said proppant. The contrast inmagnetic fields between the borehole environment and the surroundingscan give the operator an indication of the fracture internals.

The patent application publication to Knobloch (U.S. 20060037755)provides for a “Solid State Pump”. Disclosed therein is a proppantcontaining a magnetostrictive material that is moved within an internalformation such as a geological reservoir of hydrocarbon through a solidstate pumping action brought about by the introduction of a magneticsource.

The patent application publication to McCarthy et al. (US 20060102345A1) describes a “Method of Estimating Fracture Geometry, Compositionsand Articles Used for the Same”. This method involves mapping asubterranean fracture by using metallic particles with a certaindielectric constant that are contained within a proppant, and pulsingsaid particles with a particular electromagnetic radiation to performsaid mapping.

The patent application publication to Entov et al. (U.S. 20070256830),assigned to Schlumberger Technology Corporation, provides for a “Methodand an Apparatus for Evaluating a Geometry of a Hydraulic Fracture in aRock Formation”. Disclosed therein is the use of an electrolyte-basedproppant that, when injected into a borehole in a hydraulic fracturingevent, causes an electrokinetic effect from the flow of said proppantthrough the borehole aperture. The geometry of the hydraulic fracture ismapped out via the detection of electric and/or magnetic fieldstriggered by the aforementioned electrokinetic phenomenon.

The following two books are recommended to readers who may be interestedin general background information on piezoelectric and/ormagnetostrictive materials: APC International, Ltd., “PiezoelectricCeramics: Principles and Applications” (2002); and G. Engdahl (editor),“Handbook of Giant Magnetostrictive Materials”, Academic Press, New York(2000).

SUMMARY OF THE INVENTION

The present invention relates to a method for “tagging” proppants sothat they can be tracked and monitored in a downhole environment. Thisnew method is based on the use of new composite proppant compositionscontaining from approximately 0.001% to approximately 75% by volume ofdispersed fillers whose electromagnetic properties change under amechanical stress such as the closure stress of a fracture. Thesechanges of electromagnetic properties are detected by means of anysuitable technique, to track and monitor the locations of the proppants.Suitable techniques include, but are not limited to, microseismicmonitoring technology.

While the particle compositions of the invention were developed withproppant tracking applications specifically in mind, it will be obviousto workers of ordinary skill in the field of particulate materials thatsuch particles can also be used beneficially in many other applicationsby tailoring specific embodiments of the invention to meet the targetedperformance requirements of other applications.

Any suitable material (such as, but not limited to, a ceramic or apolymer) may be used as a matrix in some embodiments of the compositeproppant compositions of the invention. In some other embodiments, theingredients of a composite proppant of the invention can be agglomeratedand held together by means of a binder material. Inclusions manifestingthe piezoelectric effect or the magnetostrictive effect, or mixturesthereof, can be incorporated as dispersed fillers that serve as “tags”and thus enable the tracking of the proppant locations in a downholeenvironment. The proppants of the invention may also contain any otherdesired ingredients; including, but not limited to, rigid (mechanicallyreinforcing) fillers, impact modifiers, protective coatings, orcombinations thereof.

The imposition of a mechanical stress results in the generation of anelectric field by a piezoelectric material and in the generation of amagnetic field by a magnetostrictive material. A change in the magnitudeand/or direction of an imposed mechanical stress results in a change inthe electric field generated by a piezoelectric material and a change inthe magnetic field generated by a magnetostrictive material. The factorsgoverning the ability of a material to manifest piezoelectric ormagnetostrictive behavior are well-established. Many materials are knownto manifest such behaviors to varying magnitudes. Any of these materialsmay be used as the piezoelectric or magnetostrictive fillers in theproppants of the invention.

Strongly piezoelectric and/or giant magnetostrictive materials aresometimes significantly more expensive than the types of materials fromwhich commercial proppants are generally manufactured. There is,therefore, often a significant economic advantage to the use of blendsof proppants, where the blend includes a quantity of “tagged” proppantsthat is sufficient to produce a signal of detectable magnitude mixedwith less expensive “untagged” proppants. The use of “tagged” proppantsin such proppant blends, at amounts of at least 1% by weight of theblend, is also an aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Details will now be provided on the currently preferred embodiments ofthe invention. These details will be provided without reducing thegenerality of the invention. With the benefit of this disclosure,persons of ordinary skill in the field of the invention can readilyimagine many additional embodiments that fall within the full scope ofthe invention as taught in the SUMMARY OF THE INVENTION section.

Piezoelectric particles, magnetostrictive particles, or mixturesthereof, are dispersed in a thermoset polymer matrix in the preferredembodiments of the invention. These preferred embodiments are preparedvia suspension polymerization. They are substantially spherical inshape; where a substantially spherical particle is defined as a particlehaving a roundness of at least 0.7 and a sphericity of at least 0.7, asmeasured by the use of a Krumbien/Sloss chart using the experimentalprocedure recommended in International Standard ISO 13503-2, “Petroleumand natural gas industries—Completion fluids and materials—Part 2:Measurement of properties of proppants used in hydraulic fracturing andgravel-packing operations” (first edition, 2006), Section 7, for thepurposes of this disclosure.

The thermoset matrix used in the most preferred embodiments consists ofa terpolymer of styrene (St), ethylvinylbenzene (EVB), anddivinylbenzene (DVB) (U.S. Application No. 20070021309). The extent ofcrosslinking in these embodiments can be adjusted by varying thepercentage of the crosslinker (DVB) in the reactive precursor mixtureand/or by postcuring via heat treatment after polymerization. In onesuch embodiment, the thermoset polymer matrix may also contain adispersed nanofiller, where, by definition, a nanofiller possesses atleast one principal axis dimension whose length is less than 0.5 microns(500 nanometers). In one embodiment, the dispersed nanofiller may becarbon black, as described in U.S. Application No. 20070066491. Inanother embodiment, the thermoset polymer matrix may also contain animpact modifier, as described in U.S. Application No. 20070161515. Aprotective coating may, optionally, be applied to these embodiments. Insome embodiments, one or more of the St, EVB and DVB monomers used inthe reactive precursor mixture may be replaced by reactive ingredientsobtained and/or derived from renewable resources such as vegetable oilsand/or animal fats (U.S. Application No. 20070181302). U.S. ApplicationNos. 20070021309, 20070066491, 20070161515, and 20070181302 areincorporated herein in their entirety by reference.

The preferred embodiments use one or more of piezoelectric andmagnetostrictive fillers whose compositions cause them to manifest theseeffects very strongly. The tracking of the “tagged” proppant particlesby means of a signal that is readily distinguished from the backgroundis thus facilitated. The preferred piezoelectric fillers fall into thecategory of ferroelectric materials; defined in terms of beingspontaneously polarizable and manifesting reversible polarization, andexemplified by piezoelectric ceramics with the perovskitecrystallographic structure type such as lead zirconate titanate (PZT)and barium titanate. The preferred magnetostrictive fillers manifest“giant magnetostriction”; as exemplified by Terfenol-D (a family ofalloys of terbium, iron and dysprosium), Samfenol (a family of alloys ofsamarium and iron, sometimes also containing other elements such asdysprosium), and Galfenol (a family of alloys of gallium and iron,sometimes also containing other elements).

Different products in some of the preferred classes of piezoelectric ormagnetostrictive materials named above manifest very differenttemperature dependences for the electric field or the magnetic fieldgenerated by an applied stress. A key criterion in selectingpiezoelectric or magnetostrictive fillers for use in the preferredembodiments of the invention is that the temperature dependence of theelectric field or the magnetic field generated by an applied stressshould be as weak as possible over a downhole use temperature range ofthe proppant. In practice, piezoelectric or magnetostrictive materialsthat meet this requirement generally have (a) a Curie temperature(T_(c)) that is significantly above the maximum temperature that aproppant is expected to encounter during use, and (b) no pronouncedsecondary structural relaxations occurring between the minimum andmaximum temperatures that a proppant is expected to encounter duringuse. When a piezoelectric or magnetostrictive filler satisfies thesecriteria, the generated electric field or magnetic field can often berelated in a relatively simple manner to the location and amount of theproppant particles and to the closure stress without needing todeconvolute the effects of the temperature dependence.

The sizes of the piezoelectric or magnetostrictive fillers also affectthe performance. While filler particles of any size that can “fit into”the proppant particles can be used, there are some advantages of usingfillers that are not larger than a few microns in size. At the otherextreme, however, the piezoelectric or magnetostrictive performanceoften declines significantly when the particles become exceedinglysmall. Consequently, one often finds an optimum particle size thatdepends on the composition of the material and on the method that wasused in its preparation. The piezoelectric or magnetostrictive particlesused in preferred embodiments of the invention are powders possessing anaverage size that ranges from approximately 100 nanometers toapproximately 5000 nanometers (5 microns) depending on the particlecomposition and on how the particle was prepared. Within this range, theaverage size subrange of greatest interest for the preparation of themost preferred embodiments is from approximately 200 nanometers toapproximately 1000 nanometers (1 micron).

As a non-limiting illustrative example, we note that the “untagged”proppants (not containing any dispersed piezoelectric ormagnetostrictive materials) that are modified to obtain the mostpreferred embodiments of the invention possess a true density in therange of 1.00 to 1.11 g/cm³. (For simplicity, in all further discussion,the term “density” will be used to represent the “true density”.) Thisrange is far lower than the densities of strongly piezoelectricmaterials such as PZT and giant magnetostrictive materials such asTerfenol-D. Consequently, the density increases as the volume fractionof piezoelectric or magnetostrictive material dispersed in a proppant isincreased. In preferred embodiments, the quantity of dispersedpiezoelectric or magnetostrictive material ranges from 0.01% by volumeup to a maximum value chosen such that a proppant containing dispersedpiezoelectric or magnetostrictive materials has a density in the rangethat is commonly considered to be “lightweight” by workers in the fieldof the invention (not exceeding 1.75 g/cm³). In the most preferredembodiments, the quantity of dispersed piezoelectric or magnetostrictivematerial ranges from 0.1% by volume up to a maximum value that is chosensuch that a proppant containing dispersed piezoelectric ormagnetostrictive materials has a density in the range that is commonlyconsidered to be “ultralightweight” by workers in the field of theinvention (not exceeding 1.25 g/cm³).

The maximum volume fraction of a piezoelectric or magnetostrictivematerial for which the density of the proppant remains within thepreferred and most preferred limits of no greater than 1.75 g/cm³ or nogreater than 1.25 g/cm³, respectively, depends strongly on the densityof the piezoelectric or magnetostrictive material. Consequently, animportant general principle in the design of preferred and mostpreferred embodiments is that, when comparing candidate piezoelectric ormagnetostrictive filler materials that possess such responses ofcomparable strength (and hence of comparable detectability), it isgenerally preferable to select the material that has the lowest density.

As a non-limiting illustrative example of how the density of a dispersedpiezoelectric or magnetostrictive material may determine the maximumvolume fraction at which the density of an embodiment of the inventionreaches the upper density limits of what are commonly defined as“ultralightweight” or “lightweight” proppants, consider embodimentswhere FracBlack™ thermoset nanocomposite beads of the Sun DrillingProducts Corporation are modified by dispersing particles of the giantmagnetostrictive alloy Terfenol-D. FracBlack™ has a density of roughly1.054 g/cm³ and Terfenol-D has a density of roughly 9.2 g/cm³.Consequently, the density of an exemplary embodiment of the inventionwhere Terfenol-D particles are dispersed in FracBlack™ beads would reach1.25 g/cm³ at a Terfenol-D content of approximately 2.4% by volume(approximately 17.7% by weight) and 1.75 g/cm³ at a Terfenol-D contentof approximately 8.5% by volume (approximately 44.8% by weight).

More generally, the density, D, of an embodiment of the invention can beestimated via a simple linear relationship in terms of the volumefractions and densities of the components. If the volume fraction of theunmodified material is denoted as V_(u), then the dispersedpiezoelectric or magnetostrictive particle volume fraction equals(1−V_(u)). The relationship is D=D₁×V_(u)+D₂×(1−V_(u)) where D₁ is thedensity of the unmodified material and D₂ is the density of thepiezoelectric or magnetostrictive additive. In the specific examplegiven in the paragraph above, the calculations were carried out by usingthis equation with D₁=1.054 g/cm³, D₂=9.2 g/cm³, and D=1.25 g/cm³ orD=1.25 g/cm³, and solving for the value of V_(u), finally to obtain thevolume percentage of Terfenol-D as 100×(1−V_(u)).

In addition to reactive monomers, optional nanofiller(s), andpiezoelectric and/or magnetostrictive filler materials, the polymerprecursor mixture used in preparing the preferred embodiments of theinvention may further comprise additional formulation ingredientsselected from the group consisting of initiators, catalysts, inhibitors,dispersants, stabilizers, rheology modifiers, impact modifiers, buffers,antioxidants, defoamers, plasticizers, pigments, flame retardants, smokeretardants, or mixtures thereof.

The present disclosure may be embodied in other specific forms withoutdeparting from the spirit or essential attributes of the disclosure.Accordingly, reference should be made to the appended claims, ratherthan the foregoing specification, as indicating the scope of thedisclosure. Although the foregoing description is directed to thepreferred embodiments of the disclosure, it is noted that othervariations and modification will be apparent to those skilled in theart, and may be made without departing from the spirit or scope of thedisclosure.

What is claimed is:
 1. A composite proppant composition containing a polymer matrix and fillers whose electromagnetic properties change at a detectable level under a mechanical stress dispersed in the polymer matrix.
 2. The composition of claim 1, wherein the mechanical stress is the closure stress of a fracture.
 3. The composite proppant composition of claim 1, wherein the composite proppant composition contains from approximately 0.001% to approximately 75% by volume of the dispersed fillers.
 4. The composite proppant composition of claim 1, wherein the dispersed fillers comprise piezoelectric particles, magnetostrictive particles, or mixtures thereof dispersed in a thermoset polymer matrix.
 5. The composite proppant composition of claim 4, wherein the piezoelectric fillers comprise a ferroelectric material.
 6. The composite proppant composition of claim 4, wherein the piezoelectric fillers comprise piezoelectric ceramics with a perovskite crystallographic structure type.
 7. The composite proppant composition of claim 4, wherein the piezoelectric fillers comprise lead zirconate titanate (PZT) or barium titanate.
 8. The composite proppant composition of claim 4, wherein the composite proppant composition comprises a PZT alloy manifesting a strong piezoelectric effect or Terfenol-D manifesting giant magnetostrictive behavior.
 9. The composite proppant composition of claim 4, wherein the piezoelectric and magnetostrictive fillers are powders possessing an average size that ranges from approximately 100 nanometers to approximately 5000 nanometers (5 microns).
 10. The composite proppant composition of claim 9, wherein the piezoelectric or magnetostrictive particles are powders possessing an average size that ranges from approximately 200 nanometers to approximately 1000 nanometers (1 micron).
 11. The composite proppant composition of claim 4, wherein the piezoelectric particles, magnetostrictive particles are substantially spherical in shape.
 12. The composite proppant composition of claim 4, wherein the piezoelectric particles, magnetostrictive particles have a roundness of at least 0.7.
 13. The composite proppant composition of claim 4, wherein the piezoelectric particles, magnetostrictive particles have a sphericity of at least 0.7, as measured by the use of a Krumbien/Sloss chart.
 14. The composite proppant composition of claim 1, wherein the composite proppant composition further comprises nanofiller particles possessing a length that is less than 500 nanometers in at least one principal axis direction are dispersed in said polymer matrix.
 15. The composite proppant composition of claim 14, wherein the nanofiller particles are carbon black.
 16. The composite proppant composition of claim 1, wherein the polymer matrix is a thermoset polymer matrix containing an impact modifier. 